The proposed research is aimed at determining how the neural mechanisms controlling daily rhythms differ in nocturnal and diurnal mammals. A small group of neurons in the mammalian brain called the suprachiasmatic nucleus (SCN) is responsible for the generation of 24 hour, "circadian," rhythms. To date, most research into the neural substrates controlling circadian rhythms has been done with nocturnal rodents One reason is that there has been no suitable diurnal rodent model. The research proposed here has 3 primary objectives. First, fundamental features of the circadian system will be characterized in Arvicanthus niloticus, a diurnal rodent recently imported to the USA from Africa. A. niloticus is a hamster sized rodent that breeds rapidly in captivity and is ideally suited for studies of circadian rhythms. A. niloticus will be characterized with respect to its circadian rhythms and the neural structures controlling them. The second objective is to evaluate the hypothesis that differences in some aspect of SCN function account for differences in rhythms of diurnal and nocturnal mammals. The third objective is to test the hypothesis that diurnal and nocturnal species differ with respect to responses exhibited to signals from the SCN. The circadian system influences virtually every physiological and behavioral variable, and this influence is profoundly different in nocturnal and diurnal species. Nothing is currently known about the mechanisms underlying these differences. Although humans are diurnal, most research on circadian rhythms, as well as most medical research, is done in nocturnal rodents. It is therefore critical to identify the neural processes promoting the differences in rhythms of nocturnal and diurnal species.